Magnetic recording in its conventional form has been projected to suffer from superparamagnetic instabilities at high bit densities. As the grain size of the magnetic recording medium is decreased in order to increase the areal density, a threshold known as the superparamagnetic limit at which stable data storage is no longer feasible is reached for a given material and temperature.
Thermal stability of magnetic recording systems can be improved by employing a recording medium formed of a material with a very high magnetic anisotropy. However, very few of such hard magnetic materials exist. Furthermore, with currently available magnetic materials, recording heads are not able to provide a sufficient magnetic writing field to write on such materials.
A strategy to control media noise for high areal density recording is to reduce the lateral dimensions of the grains. The resulting reduction of the grain volume needs to be compensated by a corresponding increase of the magnetic crystalline anisotropy energy density of the media in order to ensure thermal stability of the stored bits throughout a period of at least 10 years. Although the high magnetic crystalline anisotropy of recently developed granular media like L10 FePt or CoPt supports areal densities up to several Tbit/inch2, it also hinders conventional writing.
One solution to overcome this dilemma is to soften the medium temporarily by locally heating it to temperatures at which the external write field can reverse the magnetization. This concept, known as heat assisted magnetic recording (HAMR), involves locally heating a magnetic recording medium to reduce the coercivity of the recording medium in a confined region so that the applied magnetic writing field can more easily direct the magnetization of the recording medium in the region during the temporary magnetic softening of the recording medium caused by the heat source. HAMR allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature assuring a sufficient thermal stability.
HAMR systems require the spatial and temporal variations of the heat profile to be managed. In particular, lateral heat diffusion in HAMR media is an important requirement for confining the heated region in the media to desired dimensions. Typical dimensions for Terabit per square inch recording are 25×25 nm2, assuming a bit-aspect ratio of one. If the heat delivery system delivers an intensity profile with Gaussian FWHM of 25 nm, then no additional heat spread in the media can be tolerated.
Other important aspects of HAMR are the efficiency of the heat delivery system and the cooling rate of the media. Whereas the heating has to be powerful enough to heat the media to the desired temperatures (at least close to the Curie point), the cooling rate has to be fast enough to avoid thermal destabilization of the written information during the time the media cools down. Both issues, efficiency of the heat delivery system and fast cooling rate, are mutually competitive—the faster the cooling rate the more heating power is required to achieve a certain temperature increase. The use of heatsink layers to facilitate cooling may be possible. However, known metallic materials for high thermal conductivity such as pure Cu, Ag and Al are often too soft and ductile, and they do not exhibit sufficient mechanical durability during the magnetic recording media fabrication process and during write/read operations in hard disc drives and the like.
A need therefore exits for magnetic recording media with controlled heat transfer characteristics that are durable enough to withstand magnetic recording media fabrication and recording operations.